Nitrided spring



.lune 13, 1933. y G M- EATON 1,914,083

NITRIDED SPRING Filed June 8, 1952 PEQJEC rfa Wfw dr' INVENTOR PatentedJune 13, 1933 UNITED STATES PATENT OFFICE GEORGE IMI. EATON, F BEN AVON,PENNSYLVANIA NITRIDED SPRING Application filed .Tun-e 8, 1932. SerialNo. 616,048.

ated in the critical region of the coil when subjected to stress duringcoiling, and dur. ing the thermal treatments employed as well as in thenormal Working of the spring.

The surface defects are of a nature peculiarly susceptible tomagnification durin the rapid heat cycle attendant upon a quenc ingoperation and the primary object of this invention is to prevent orsubstantially reduce the exaggeration of the surface defects in helicalsprings.

I have found that by relieving the steel of the shock from quenching,the conditions which are mainly responsible for the epidemic` failuresof helical springs are substantially removed.

The type of failures herein referred to and the conditions that causethem may be better understood in connection with the accompanyingdrawing constituting a part hereof, in which like reference charactersdesignate like parts, and in which:

Figure 1 is a side elevational viewpartially in section of a typicallcoil spring showing the type of failureto be eliminated or delayed; 1

Figure 2 is a view diagrammatically illustrating a stress concentrationat the inside of coil; 21nd Figure 3 illustrates cross-sectional viewsshowing ideal structure distribution when produced without local stressin accordance with this invention and structure distribution obtained byconventional heat treatment.

I employ a steel, the analysis of which lends it to nitriding to protectthe surface of the 0 spring against atmospheric corrosion and which atthe same time is adaptable to air hardening` to a degree which givesadequate characteristics .to the material in the core of the spring rodor wire.

I have discovered that alloy steels of the following analysis aresuitable for this purpose:

Carbon 0.15 to 0.35 Manganese '4 0.40 to 0.80 60 Aluminum 1.00 to 2.50Molybdenum 0.40 to 1.00 Sulphur Low. Phosphorus Low Silicon Normal Thesteel rod is centerless ground, then coiled to the size and shape of thehelical spring desired, as shown in Figure l, and is then subjected to anormalizing treatment in a non-decarburizing atmosphere to relieve thecoiling stresses if essential. The centerless grinding of the spring rodremoves or reveals the surface seams or streaks designated by theparallel arrows in Figure 1 of the drawing and prepares the surface toreceive the nitrided case. The coil is then nitrided by heating it in anatmosphere of ammonia gas at a temperature of about 10000 F. for asuicient period of time to produce a case of suitable depth. Thermallythe nitriding treatment constitutes a drawback from the normalizingtreatment and leaves the steel in a strong and tough condition quiteideal for the core of the spring rod. Thev slow rate of cooling involvedin the nitriding process produces no transient surface tension withdanger of incipient or actual cracking as in quenched springs.

In making the highest quality springs it may be desirable vto subjectthe material to a thermal pre-treatment for structural refinement, andthe spring stock should have its decarbonized surface removed as by cenjterless grinding before subjecting it to the nitriding process.Structural refinement is for the double purpose of adapting the steel totake the nitriding treatment, and to resist service stresses. Centerlessgrinding may be omitted for lower grade springs if the laylo er ofdecarbonized material is of immaterial thickness, but any materialdecarburized layer will damage the character oi? the nitrided case.

The nitrided spring possesses an outer shell of extreme hardness anduniform depth as shown in the section marked (a) in Figure 3 of thedrawing which has such toughness that the shell will resist crackingeven when the entire body is subjected to some degree of permanentdistortion and which possesses an elastic limit far above that of anyother steel which has been heat-treated at a temperature not exceeding1000o F. Such springs be* cause of their smooth hard shell free fromincipient surface flaws or cracks can be Worked close to the elasticlimit of the material in shear since the shearing stress Will not beconcentrated on a Weak surface portion to cause failure.

The spring can be accurately formed prior to nitriding, and in precisionsprings when they are of correct shape and free from internal stress,the spring after hardening Will be of substantially the identical shapeas the unhardened spring.

The sections (b) and (c) of Figure 3 show structure distribution whichare the best that heat treatment can produce as compared with thestructure distribution shown by section (a) which is produced by theprocess herein described.

Although l have specified molybdenum alloys as particularly adapted forhelical springs it will be obvious that other steels suitable fornitriding, may be employed Within the spirit and scope of my invention.

l claim:

1. A nitrided steel helical spring containing by analysis:

Carbon 0.15 to 0.35 Manganese 0.40 to 0.80 Aluminum 1.00 to 2.50Molybdenum 0.40 to 1.00

2. As an article of manufacture a nitrided steel helical spring.

ln testimony whereof I have hereunto set my hand.

GEORGE M. EATON.

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